The present invention relates to a method for measuring alternating electrical current in a current conductor by a Faraday element, and to a device for implementing the method. A method of this type is disclosed, e.g., in U.S. Pat. No. 4,755,665.
Optical measuring devices for measuring electrical current in a current conductor using the Faraday effect are known. These devices are also described as magneto-optic current transducers. The Faraday effect is understood to be the rotation of the plane of polarization of linearly polarized light in dependence upon a magnetic field. The angle of rotation is proportional to the path integral along the path covered by the light over the magnetic field, using the Verdet constant as a proportionality constant. The Verdet constant is dependent upon the material through which the light is passing, its temperature, and the wavelength of the light. To measure the current, a Faraday element, which is made of an optically transparent material that demonstrates the Faraday effect (generally glass) is arranged in proximity to the current conductor.
Linearly polarized light is transmitted by a transmitter unit through the Faraday element. The magnetic field produced by the electrical current causes rotation of the plane of polarization of the light in the Faraday element through a polarization angle of rotation. This polarization angle can be evaluated by an evaluator unit as a measure of the strength of the magnetic field and, thus, of the intensity of the electric current. The Faraday element generally surrounds the current conductor, so that the polarized light circulates around the current conductor in a quasi-closed path. As a result, the size of the polarization angle of rotation is roughly directly proportional to the amplitude of the measured current.
In one known specific embodiment disclosed in EP-B1-0088419, the Faraday element is designed as a solid glass ring which is disposed around the current conductor. In this specific embodiment, the light circulates around the current conductor only once.
In another known specific embodiment, the Faraday element is designed as part of an optical monomode fiber, which surrounds the current conductor and is in the form of a measuring winding. Thus, for one pass-through, the light circulates around the current conductor N-times, where N is the number of turns of the measuring winding. Two types of such magneto-optic current transducers having optical-fiber measuring windings are known: the transmission type and the reflection type. In the transmission type, the light is input to one end of the fiber and is emitted from the other end, so that the light only passes through the measuring winding once. On the other hand, in the reflection type, light is input to one end of the fiber and the other end of the fiber is provided with a reflective surface, so as to reflect the light. This causes the light to pass through the measuring winding a second time in the opposite direction, and to be emitted at the first end. Due to the non-reciprocity of the Faraday effect, when the light is passed through in the opposite direction, the plane of polarization of the light is rotated again by the same amount in the same direction as occurred during the initial pass-through. Thus, given the same measuring winding, the angle of rotation in a reflection type arrangement is twice as large as in the case of the transmission type. As disclosed in WO91/01501, a beam splitter may be provided to separate the light that is input to the fiber from the light that is emitted out of the fiber.
In addition, methods are known for evaluating the information contained in the rotated plane of polarization of the measuring light over the measured current. Further, corresponding devices for implementing these methods are known, in which, in principle, all the specific embodiments of Faraday elements can be utilized.
One problem encountered in all magneto-optic current transducers is the interference effect caused by linear birefringence (i.e., double refraction) in the Faraday element and the optical transmission paths. Linear birefringence of this type can be caused by mechanical stresses in the material. These stresses may be caused, for example, by bending, vibration or, in particular, by temperature variations.
In the magneto-optic current transducer disclosed in EP-B1-O 088 419, the light from a light source is linearly polarized by a polarizer and then coupled into the Faraday element. The linearly polarized light passes through the Faraday element and is emitted. The emitted light is split by a Wollaston prism, which acts as an analyzer, into two linearly polarized light signals A and B with planes of polarization disposed at right angles to one another. These two light signals A and B are transmitted via corresponding optical transmission fibers to corresponding light detectors and are converted into corresponding electrical signals PA and PB. From these two signals PA and PB, a Faraday angle of rotation is calculated as a measuring signal by a computing unit. This measuring signal corresponds to the expression (PA-PB/PA+PB), which is the quotient of the difference between the two signals over the sum of the two signals. The use of this quotient compensates for different sensitivities of the light detectors and different damping factors for the intensities of the light signals A and B in the two transmission fibers. However, the use of the quotient does not compensate for temperature effects.
In another known evaluation method, the two signals PA and PB are each subdivided by a filter into their direct-current components PA(DC) and PB(DC) and their alternating-current components PA(AC) and PB(AC). For each of the signals PA and PB, the quotient QA=PA(AC)/PA(DC) or QB=PB(AC)/PB(DC) is formed from the alternating-current components PA(AC) or PB(AC) and from the direct-current component PA(DC) or PB(DC) to compensate for varying light intensities caused by fluctuations in transmission and sensitivity. From each of these two quotients QA and QB, an average time value Q and QB is generated and, finally, from these two average values QA and QB, a quotient Q=QA/QB is formed. Within the framework of an iteration method, a comparison is made with standardized values stored in a table of values (look-up table) to obtain a correction factor K for the calculated quotient Q. The value Q.times.K, corrected by this correction factor K, is retrieved as a temperature-compensated measuring value for the measured current. This method makes it possible to reduce the temperature sensitivity of the magneto-optic current transducer by about 50 times. (Proc. Conf. Opt. Fiber Sensors OFS 1988, New Orleans, pp. 288-291 and U.S. Pat. No. 4,755,665). However, the above-described iteration method is quite expensive.